Pythium spp.

(Damping-off of seedlings)
 

Unlike to what is reported in other countries, Pythium spp. does not seem to be a serious thread for tobacco nurseries in France. Floating seedbeds, although seem much more vulnerable as seen in the U.S., have not yet been contaminated in France. This may be due to anti-mildew treatment made in the nursery. Treatments, particularly those with mefenoxam combinations, can probably control the development of fungi-like organisms, classified in chromista. Mefenoxam is indeed a very powerful anti-Pythium "fungicide". During the first treatments, it is possible that part of the product pours out and partially soaks the substrate in the trays. 

Pythium spp. cause soft and damp lesions at the collar level and sometimes browning of roots. The presence of sporangia or oospores (figure 1.) is easily observed under the microscope in damaged tissues. Several species of Pythium spp. are likely to attack tobacco: P. ultimum Trow (1901), P. aphanidermatum (Edson) Fitzp. (1923), P. debaryanum R. Hesse (1874), P. butleri Subramani. (1919), P. monospermum Pringsh. (1858), P. deliense Die (1934), P. myriotylum Drechler (1930), P. splendens Hans Braun (1925), P. vexans de Bary (1876), P. perniciosum Serbinow (1912), etc.. A certain number of these species are present in France and have been observed in other cultivated crops. One should be particularly vigilant because these fungi are a real threat to the production system. Heating of the nutrient solution provides comfort to the seedlings but it may also change the behaviour of the fungi.
 
For a better understanding of these aquatic micro-organisms, you can see the fact sheet of The oomycetes.